Phosphorescent Devices

Abstract

Incoherent light sources depending on phosphors which may simultaneously emit at more than one wavelength are provided with multiple dielectric coatings to suppress a portion of the emission and thereby enhance the remainder. The use of such coatings with frequency up-converting phosphors as well as down-converting phosphors is described.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is concerned with incoherent light sources utilizing phosphor emission.

2. Description of the Prior Art

Incoherent light sources based on phosphor emission are already in prevalent use and many new uses are contemplated. Such sources depend upon a variety of pump means as, for example, electron bombardment in cathode ray tubes; d.c. electric biasing in junction devices, such as those using gallium arsenide; and light pumping as in a variety of display devices. The latter category includes higher frequency pumping in most common devices and lower frequency pumping as in second photon devices. See Bulletin of the American Physical Society, Series 11, Vol. 13, No. 4, p. 687, Paper HK7.

Phosphor materials are of many types, some inorganic, some organic; some emit over rather narrow bandwidths, some over broad bandwidths.

In any of the foregoing categories, a situation may arise in which part of the pump energy is converted to undesired emission. This undesired emission may be within or without the visible spectrum. A specific example of recent concern has to do with second photon sources utilizing long wavelength pumps. In one such example, a forward biased GaAs diode is used to pump a rare earth-containing, second photon phosphor to produce visible emission. Whereas such devices operate efficiently at green and red wavelengths, difficulty has been encountered in fabricating an efficient blue source. In this particular example, a blue source is desired for the construction of a three-color display system. While thulium-containing materials (the initial absorption function being performed by ytterbium) emit blue light when pumped by the infrared emission from the diode, a significant part of the pump energy is converted to a different wavelength of near infrared emission. As a result, the efficiency of conversion to blue is diminished. Many other similar examples exist.

A further complication resulting in inefficiency in phosphorescent devices is concerned with inefficient utilization of pump energy. In light-pumped devices, absorption coefficients for different involved wavelengths may dictate different optimum thicknesses for emission and for pump energy. Under some circumstances, for example, dimension optimization for emission may result in inefficient absorption of pump energy.

Problems similar to many of the foregoing were a deterrent to the development of the laser. The problem there was largely one of absorbing sufficient pump energy to create the required population inversion. Resort was had to layered structures of various dielectric films all individually transparent to wavelengths of concern. Choice of thickness of two or more materials of appropriate refractive indices results in constructive and destructive interference at selected wavelengths.

This approach has permitted the design of a cavity which is essentially totally resonant for the pump frequency. Energy of the wavelength of concern may also be resonated so as to give the required statistical number of passes for desired operation. See Applied Optics and Optical Engineering, ed. R. Kingslake, Academic Press, New York, 1965, Ch. 8.

SUMMARY OF THE INVENTION

In accordance with the invention, multilayered coatings of transparent materials of critical thickness and refractive indices partially or totally encompassing phosphor materials result in suppression of energy of one or more wavelengths while permitting transmission of energy of one or more other wavelengths. This is a general solution which results in improvement of efficiency of incoherent phosphorescent devices in any of the classes set forth above. In certain embodiments, pumping efficiency is improved by preventing escape of part of the pump energy or even by creating resonant conditions for such pump energy. In the preferred embodiment, significant improvement in emission is brought about by suppression of one or more emission wavelengths to enhance at least one other wavelength in phosphors having relevant emission spectra.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an energy level diagram in ordinate units of wavenumbers for an appropriate second-photon phosphor system illustrative of systems suitable for improvement in accordance with the inventive principles;

FIG. 2 is a sectional view of a structure showing improved emission efficiency in accordance with the invention;

FIG. 3 on coordinates of transmittance in percent, and wavelength in microns illustrates the relationship of these coordinates for a particular layered structure;

FIG. 4, in ordinate units of wavenumbers is an energy diagram illustrating a down-converting phosphor system with multiple emission lines, the efficiency of which may be improved in accordance with the invention;

FIG. 5 is a sectional view of a phosphor layer dielectrically coated in accordance with the invention; and

FIG. 6 is a sectional view of a portion of a structure alternative to that of FIG. 5.

DETAILED DESCRIPTION

The invention has been generally described. The state of the concerned arts is such that further description is unnecessary to enable the person skilled in the art to practice the invention. Suitable dielectric materials, relevant dielectric layer parameters including refractive indices and thicknesses for accomplishment of suppression and transmission as desired are available in the literature. See for example Applied Optics and Optical Engineering, ed. by R. Kingslake, Academic Press (1965), Vol.II, Ch. 8.

For illustrative purposes, a detailed description is set forth in terms of the ytterbium-thulium, second-photon phosphor. This particular system is of interest as a blue light source, for example, as an indicator light or a portion of a display screen with light pumping at a suitable infrared wavelength. Since absorption is relatively narrow, this material is particularly suitable for use with a narrow band emitting pump such as a laser or a forward biased incoherent diode. The prime example of the latter at this writing is the gallium arsenide diode.

1. Drawing

FIG. 1. In the ytterbium-thulium system (suitable hosts include yttrium fluoride), infrared excited blue emission is produced by a three-step sequential excitation. The efficiency of the infrared excited blue emission from level 3 (all levels encircled on the figure) is approximately 0.1 percent (i.e.

). At present, the blue emission is limited to this low value because significant emission at 8,000 A. from level 2 occurs. In fact, the emission from 8,000 A. is from 4 to 10 percent efficient. A technique to improve the blue emission at the expense of the 8,000 A. emission is to provide a reflective coating on the phosphor so as to effectively increase the radiative lifetime of level 2, thus increasing the probability of excitation of atoms to level 3 as compared to the probability of the 8,000 A. radiative transition. In Tm, the 8,000 A. transition occurs to the ground state; and in this case, if a coating of reflectivity R is used, the effective radiative lifetime can be increased to

where .tau. is the normal radiative lifetime of the Tm.sup.3.sup.+ 2 level. Since with multilayer coatings a reflectivity of greater than 90 percent is easily achievable, emission at 4,800 A. (blue) is increased by at least a factor of 10.

Several methods of entrapping the 8,000 A. radiation to improve the blue emission are discussed in FIGS. 2 and 3. In FIG. 2 phosphor 1 such as YF.sub.3 :Yb,Tm is in the form of a thin transparent coating on the diode 2 which may be Si-GaAs. The dome surface 3 of the diode and the outer surface 4 of the phosphor have been coated with a multilayer coating which is reflective at 8,000 A. and transparent at 4,800 A.

A fifteen-layer coating which can be used is represented in FIG. 3. The coating consists of a thirteen-layer, 1/4.lambda.(.lambda.=0.57.mu.), high and low index stack in which the high index layer H.dbd.ZnS and the low index layer L.dbd.MgF.sub.2. On either end, a 1/8.lambda. layer (H/2) of the high index material is used. The general characteristic of such a coating is also shown. If also the coatings are partially reflective at the pump frequency (0.93.mu. for GaAs diodes) one can get an even further enhancement because the intensity of the 0.93.mu. radiation in the phosphor is effectively increased by a factor proportional to the number of internal reflections. The enhancement of the efficiency of conversion to blue light (4,880 A.) is proportional to the N-1 power of the 0.93.mu. intensity where N is the number of sequential photons involved providing saturation effects have not been reached. N=3 for 0.480.mu. emission.

Enhancement at 8,000 A is discussed. The coating is highly reflecting at 4,880 A if the layers of the same dielectric structure described above are 1/4.lambda. at 2,800 A. or 700 A. thick. Such a coating reflects 4,800 A. and transmits 8,000 A. and 0.93.mu.. Thus the Tm phosphor can be used to pump YAG:Nd which absorbs at 8,000 A. without undue loss as blue emission (4,880 A.). Normal operation is 300 Amperes/cm.sup.2 in a GaAs diode.

Alternative ions emitting in the visible are Er, Ho. Devices are again provided with coatings that reflect at all emission energies save the one desired. It is important to provide suitable reflection particularly for undesirable emissions having short intrinsic radiative lifetimes.

Suitable host materials and other considerations germane to the design of efficient light sources of the type described in conjunction with FIG. 2 have been set forth elsewhere, see Applied Physics Letters, Volume 15, No. 2, pages 48 to 54. Host materials may be simple fluorides or more complex media shown to enhance operation in accordance with a variety of mechanisms.

The energy diagram of FIG. 4 is illustrative of a more conventional phosphor which emits at several wavelengths .lambda..sub.1, .lambda..sub.2, and .lambda..sub.3 all longer than the pumping wavelength. If any one of these fluorescences, say .lambda..sub.2, is preferred, emission at that wavelength is improved by the suppression of emission from the phosphor at the undesired wavelengths .lambda..sub.1 and .lambda..sub.3 using multilayer coatings on the phosphor which are highly reflective at the undesired wavelengths and transmitting at the desired wavelength. While the concept and the diagrams are general and apply to a large number of conventional phosphors, a specific example is a phosphor containing the active ion Er.sup.3.sup.+ in which case .lambda..sub.p is a band of wavelengths from 0.5 - 0.4.mu. and .lambda..sub.1 =0.55.mu., .lambda..sub.2 =0.65.mu. and .lambda..sub.3 =0.82.mu.. For this case, the pump may be a conventional Hg-Arc source.

FIG. 5 depicts a phosphor layer 10 covered by coatings 11 and 12. Coating materials are selected in accordance with the considerations set forth above.

In FIG. 6, the phosphor material 15 is particulate and each particle is coated with multiple layers 16 to accomplish the end described. While present techniques do not produce coatings of the thickness uniformity which may be accomplished on massive smooth surfaces, procedures are available for producing coatings which, while they may not optimize, nevertheless improve emission efficiency. Such techniques include evaporation, sputtering and various other deposition techniques.

2. Design Requirements

The general requirement of the invention is that at least one emitting surface of a phosphor be contacted by at least two layers of materials of differing refractive indices so chosen as to unequally suppress a portion of the spectrum relative to another such portion. Suitable materials are necessarily transparent to all concerned wavelengths, it being considered that an absorption of 5 percent at any concerned wavelength is the maximum permitted. The number of layers, their indices and thickness, all depend on the particular circumstances involved.

It is known that the applicable principles are those of conventional filter design. Where it is desired to suppress or transmit a relatively broad bandwidth to a relatively uniform degree, a large number, for example fifteen or more layers may be required. In less sophisticated devices where it may suffice merely to suppress one or more relatively narrow bands and/or where flat response is of little consequence, a smaller number of layers, as few as two, may suffice.

Claims

We claim:

1. Incoherent phosphorescent emission source comprising a phosphor adapted to at least partially transmit electromagnetic radiation of different wavelengths, characterized in that said phosphor is provided with a medium at least partially encompassing said phosphor, said medium consisting essentially of at least two successive layers, said layers being of such thicknesses and having such refractive indices as to suppress one of the said wavelengths relative to the other in which said phosphor is of such nature as to produce at least one wavelength which is shorter that that of a pump.

2. Source of claim 1 in which the wavelength of the said pump is in the infrared spectrum.

3. Source of claim 2 in which the said pump is a forward biased, gallium arsenide diode.